1. Field of the Invention
The present invention relates to a technology of detecting an input break of differential M-phase shift keying signal light in an optical communication system, where M=2n (n is a positive integer).
2. Description of the Related Art
Conventionally, in optical communication systems as typified by the Internet of recent years, use of an optical communication system using a differential quadrature phase shift keying (DQPSK) is considered as a communication method in which frequency utilization efficiency is improved, to respond to rapidly increasing demand for data communication. Moreover, in the optical communication system, an input-break detecting circuit to prevent an improper operation caused by a noise originated in an input break of a DQPSK signal light has been used (for example, Japanese Patent Laid-Open Publication No. H3-79141).
FIG. 5 is a block diagram of a conventional optical communication system. As shown in FIG. 5, a DQPSK signal light transmitted by a transmitting device (Tx) 510 is multiplexed by a multiplexing unit 520, and input to a demultiplexing unit 540 through a repeater 531 and a repeater 532. The multiplexed DQPSK is demultiplexed by the demultiplexing unit 540 and received by a receiving device 550. The receiving unit 550 includes an amplifier 551, an input-break detecting circuit 552, a virtually imaged phased array (VIPA) as a dispersion compensator 553, an amplifier 554, a demodulator 555, a data processing unit (FRAMER+FEC) 556.
The input-break detecting circuit 552 detects an input break of a DQPSK signal light by monitoring an average value of power of light input to the amplifier 551. The demodulator 555 includes a delay interferometer and a phase-error detecting circuit to detect a phase error in a control phase amount, and demodulates the DQPSK signal light. The data processing unit 556 performs a logic processing such as error correction, based on a data signal demodulated by the demodulator 555.
FIG. 6 is a flowchart of an operation of a conventional receiving device. As shown in FIG. 6, first, it is determined whether power of an input light is equal to or higher than a threshold (step S601). When the power is lower than the threshold (step S601: NO), it is determined as an input break of the DQPSK signal light. Thus, a series of processes is ended. When the power is equal to or higher than the threshold (step S601: YES), a phase amount of the delay interferometer in the demodulator 555 is controlled (step S602).
Next, it is determined whether an output value of the phase-error detecting circuit is within a predetermined range (step S603). When the output value of the phase-error detecting circuit is not within the predetermined range (step S603: NO), the process returns to step S602, to continue processing. When the output value is within the predetermined range (step S603: YES), a control of a dispersion compensation amount and a logic processing are performed (step S604).
Subsequently, it is determined whether a bit error rate (BER) of the data signal is equal to or higher than a threshold (step S605). When the BER is lower than the threshold (step S605: NO), the process returns to step S604 to continue processing. When the BER is equal to or higher than the threshold (step S605: YES), a normal operation is started (step S606), and a series of processes is ended.
However, in the conventional technique described above, the input-break detecting circuit 552 monitors the average value of power of the input light without distinguishing the DQPSK signal light from a noise component. Therefore, even when the input light is a white light such as amplified spontaneous emission (ASE) light not including the DQPSK signal light, if the power of the white light is high, an input break of the DQPSK signal light cannot be detected.
Furthermore, when only white light is input, an output voltage becomes 0 volt (V), and the phase-error detecting circuit of the demodulator 555 cannot distinguish this from a case where the control phase amount for the DQPSK signal light is appropriate. Accordingly, although the DQPSK signal light is not properly received, the operations at steps S601 to S606 are performed, and the BER is not equal to or higher than the threshold. As a result, the operations at steps S604 and S605 are endlessly repeated.
Thus, the receiving device 550 becomes out of control. In addition, because the control of the dispersion compensation amount is continuously repeated, the dispersion compensator 553 can break down. Moreover, to determine whether the DQPSK signal light is included in input light, an instrument for measuring waveforms of the input light, such as a spectrum analyzer, is separately required.